[Closed] Postage variations

With ordering so much on line during lockdown, I have been struck with the large variations in postage charges. Buying odds and ends off eBay ,cables, valve cores, ferrules, are usually free despite the item costing only a few quid. Yet buying larger more expensive items often costs a fiver. Some shops do free over£20 ,some over £50, some even £100. It has definitely put me off buying some mid range items when I have got to the basket and discovered it was going to cost a third of the item cost in postage.
So why the huge variation?